1. Field of the Invention
This invention relates to an improved method for the electrodeposition of lithium from a nonaqueous solution which comprises a solution of at least one lithium salt in liquid sulfur dioxide. More particularly, it relates to the use of a triglyceride of ricinoleic acid as an additive for such a solution.
2. Description of the Prior Art
A substantial amount of interest has recently been centered on the development of ambient temperature, high energy density, electrochemical cells which are light in weight and capable of providing a higher voltage than conventional cells such as nickel-cadmium and lead-acid systems or alkaline cells having zinc anodes. The high energy density cell systems which are currently of interest typically involve the use of active metals (metals above hydrogen in the electromotive series of elements which are unstable in an aqueous environment) as anodes in combination with nonaqueous electrolytes. As used herein, "nonaqueous" is intended to mean substantially free of water. Lithium has been of particular interest as an active metal for such high energy density cells since it is the most active of the metals in the electromotive series and has the ability in an electrochemical cell to provide the highest performance in watt-hours per kilogram of all known active metals.
In conventional electrochemical cells, cathode depolarizers are used in a form which will permit an intimate and maximum contact with an external electrical circuit, such as a set of wires connecting the electrodes of a cell, while also effecting a physical separation of the cathode depolarizer from the anode. In such cells, the cathode depolarizer is generally an insoluble, finely divided solid which is either admixed with or used as a coating over an inert conducting material, such as nickel, graphite or carbon rod, which serves as a current collector or cathode. The physical separation of the cathode depolarizer from the anode is necessary to prevent a direct chemical reaction between the anode material and the cathode depolarizer which would result in self-discharge of the cell.
Until recently, it was generally believed that a direct physical contact between the cathode depolarizer and the anode could not be permitted within an electrochemical cell. It has been discovered, however, that certain cathode depolarizers do not react chemically to any appreciable extent with active metal anodes at the interface between the anode and the cathode depolarizer. Accordingly, with materials of this type, it is possible to construct an electrochemical cell wherein an active metal anode is in direct contact with the cathode depolarizer. For example, U.S. Pat. No. 3,567,515 issued to Maricle et al. on Mar. 2, 1971, discloses the use of sulfur dioxide as a cathode depolarizer in such a cell in combination with a lithium anode.
During the charging of a rechargeable lithium-sulfur dioxide electrochemical cell, lithium cations in the electrolyte are reduced at the lithium electrode and are electrodeposited onto the electrode as lithium metal. Ideally, the electrodeposited lithium is laid down as a smooth layer over the entire electrode surface, thereby preserving the electrode surface morphology from one discharge-charge cycle to the next. In practice, however, the lithium tends to deposit preferentially at certain sites on the electrode. As a consequence, the morphology of the lithium deposit is such that the electrode surface undergoes modification ranging from moderate roughening to formation of a coating of filaments or dendrites over the entire surface. After several cycles, the electrode can become covered by a dense mat of interwoven dendrites. This type of lithium deposition is undesirable and also hazardous in electrochemical cells because the lithium dendrites are often small enough to penetrate the microporous materials that are conventionally used to separate the lithium anode from the cathode current collector. As a consequence, the dendrites can grow through the separator material and cause a short-circuit between the electrodes, resulting in cell failure and possible explosion. Dendrite growth around the edges of the separator material can also occur with similar results.
The electrodeposition of lithium from a nonaqueous solution which comprises a solution of at least one lithium salt in liquid sulfur dioxide is disclosed, for example, in U.S. Pat. Nos. 3,493,433 (Feb. 3, 1970) and 3,953,234 (Apr. 27, 1976), both issued to Hoffmann.
U.S. Pat. No. 3,953,302, issued to Rao et al. on Apr. 27, 1976, discloses a method for suppressing dendrite formation during the electrodeposition of lithium from a nonaqueous solution which, if desired, can contain sulfur dioxide. This method involves incorporating into the solution an additive which contains a metallic element that is capable of coplating with lithium.
U.S. Pat. No. 1,826,159, issued to Westbrook on Oct. 6, 1931, discloses that an improved cadmium deposit can be obtained from a cadmium-cyanide electroplating bath through the use of sulfonated castor oil as an additive. Similarly, U.S. Pat. No. 4,014,761, issued to Passal on Mar. 29, 1977, discloses the use of sulfonated castor oil as a component of an aqueous electroplating bath for the electrodeposition of zinc. However, neither of these patents suggests the use of chemically unmodified castor oil as a component of a nonaqueous electroplating bath for the electrodeposition of lithium.